Foliar endophytes and ectomycorrhizal fungi alter defensive chemistry of western white pines with potential consequences on disease resistance

Background/Question/Methods

Hundreds of asymptomatic fungal taxa live inside healthy white pine tissues. Recent studies suggest that these fungi can influence the frequency and severity of infections by fungal pathogens such as Cronartium ribicola, the causal agent of white pine blister rust. In a full-factorial experiment, we inoculated western white pine (Pinus monticola) from six seed families with foliar fungal endophytes (FFE) or ectomycorrhizal fungi (EMF, genus: Suillus), as well as a combined fungal treatment (FFE+EMF), and a control treatment (no inoculation). The six seed families consisted of half-siblings and full-sib progeny, with both high and low levels of known disease resistance. Four months post-inoculations, we infected a subset of seedlings from all four treatments with the rust to determine the individual and shared effects of FFE and EMF on resistance to white pine blister rust. We measured tree defensive chemistry (terpenes) for all treatments immediately before, and four months after infection, as disease symptoms began to appear.

Results/Conclusions

Seed family influenced both initial disease severity and terpene composition in all seedlings. Terpene composition did not differ among fungal treatments immediately before pathogen infection. However, there was a strong treatment effect on terpenes after pathogen infection (p < 0.005), indicating that fungal inoculations altered seedlings’ induced defense responses. EMF inoculations had the greatest influence, while EMF+FFE treated seedlings were more similar to untreated control seedlings, suggesting an interaction between above and below ground fungi. How these differences in induced defensive responses translate to disease resistance will be assessed with ongoing monitoring of disease progression.